1. Field
Various embodiments described herein relate to a demodulator.
2. Description of the Related Art
A demodulator splits a beam of light, provides the resulting beams with a time difference corresponding to one bit, and recombines them, in order to demodulate a signal by using a one-bit phase difference of light (see, Japanese Laid-open Patent Application Publication No. 2007-67955). In micro-optics, a difference is provided in the space propagation time. In this case, a beam of light propagating through a space increases in beam diameter due to diffraction. The longer the wavelength of propagating light, the more susceptible to diffraction it is. The smaller the beam diameter of propagating light, the more susceptible to diffraction it is.
However, the wavelengths used in fiber-optic communication are prescribed, and the wavelength cannot be freely selected. In order to prevent the beam divergence, the distance for which parallel light can be maintained can be increased by increasing the beam diameter.
However, increasing the beam diameter necessitates increasing the sizes of optical elements. In addition, the distance between the fiber end face and the lens needs to be increased. This increases the entire size of the device.
In recent years, communication modules have been reduced in size, and reduction in size of devices is strongly required. When a demodulator combines small-diameter beams that travel different distances to provide a necessary delay, residual light is generated due to the difference in beam diameter even when the power of light is desired to be zero. This deteriorates the extinction ratio (the on-off ratio).
In a demodulator, deterioration of extinction ratio leads to an increase in noise. Consequently, the extinction ratio is an important performance value specified in the requirement specifications. When a lens or the like is used to match different beam diameters, the phase front differs between two beams to be combined, and this also deteriorates the extinction ratio. Therefore, it is required to use thin beams, to match the beam diameters, and to match the phase fronts.